1. Field of the Invention
The present invention relates to a combination of a piston and a piston ring, or a piston assembly, which is applicable to a high-powered internal combustion engine, and more particularly applicable to a high-powered internal combustion engine such as a diesel engine operated at high temperature and high combustion pressure.
2. Description of the Related Art
In a conventional diesel engine, there has been generally used a combination of a piston made of aluminum or a piston made of aluminum having a wear-resistant ring (so called, “ring carrier”) and a piston ring having a hard film formed at an outer peripheral sliding surface of the piston ring, the piston ring being subjected to a nitriding treatment as occasion demands.
However, in recent years, a demand for realizing a high-powered internal combustion engine has been increasing from the point of view of environmental protection. Under such high-powered operation accompanying high combustion temperature and high combustion pressure, the aforementioned aluminum-based piston has been durability, heat resistance and the like. Accordingly, there has been investigated a piston made of steel capable of improved performance even under high-powered operating conditions.
However, where the aforementioned piston made of steel and the piston ring made of steel is adopted, a lower surface of the piston ring is liable to stick to a lower surface of the piston ring groove due to friction heat. When such a phenomenon is observed, there has been a possibility of problems such that a following-up property of the piston ring with respect to a cylinder liner would be remarkably lowered, so that the functions inherently required for the piston ring cannot be achieved.